The chronically catheterized fetal sheep with indwelling vascular catheters will be used as an experimental model to study the endocrine effects, synthesis, and secretion of prolactin by the fetus. Detailed studies of plasma concentrations will be made in individual animals over this period and especially during the critical period of augmented adrenal function that is known to trigger delivery in this species. Use will be made of hypophysectomized fetal sheep as well as fetal preparations in which fetal pituitary stalk section has been performed to perturb prolactin levels. Investigation of the kinetics of prolactin production will be undertaken in fetal placental circulation. Basal function and changes in the level of activity of the hypothalamic-pituitary prolactin system will be studied to assess the possible role of fetal prolactin in fetal adrenal and placental steroidogenesis. The possible role of fetal prolactin and the other systems with which it interacts are to be studied as they relate to the initiation of parturition in this species. Infusions of NIH-ovine prolactin into intact, hypophysectomized and stalk-sectioned fetal sheep will be carried out, together with an assessment of the major glucocorticoids in this species, cortisol and corticosterone. Also the interrelationship of prolactin and fetal progestins, estrogens, androgens, and their sulfoconjugates will be studied. Finally, we plan to study maternal prolactin secretion and its relationship to secretion of maternal corticosteroids, estrogens, and progestins, both during normal delivery and during premature delivery induced by the infusion of cortisol to the fetus at physiological rates.